Pacemakers are devices implanted in the body to stimulate the heart and regulate its contractions. Many pacemakers have housings which are implanted in the patient's chest or abdomen, and which have leads extending to the heart, wherein electrodes on the leads apply electrical stimulation. The pacemaker typically applies electrical stimulation having a fixed amplitude, one which is chosen with a safety margin that ensures successful “capture,” that is, the myocardium exhibits the desired behavior (depolarization) after receiving stimulation (a pacing pulse).
Leadless pacemakers, wherein electrodes are provided directly on the housing, and the housing is sized to be implanted entirely within a chamber of the heart (see U.S. Pat. No. 9,358,387), are growing increasingly popular. A typical design has a pacing electrode at one end, and the housing (the “can”), or a portion thereof, is connected to battery ground so that it can serve the role of the return electrode. Because leadless pacemakers must be made very small for optimal operation—typically smaller than a AAA battery—they are subject to severe engineering constraints regarding the size (and thus capacity) of their components. Because their batteries must be small, the batteries typically have more limited energy storage capacity. One way for a leadless pacemaker to conserve energy, and thus prolong its battery life, is to avoid use of the aforementioned “excess stimulation,” and instead utilize an automatic capture control algorithm which finds the minimum stimulation needed to attain successful capture. This is difficult owing to constraints regarding the size and computing ability of its electronics module. In conventional (lead-bearing) pacemakers, automatic capture assessment is typically done by measuring evoked response (ER), which is the electrical signal generated by the depolarization of myocardial cells following a pacing pulse. It is known to be challenging to reliably measure an atrial ER in response to an atrial pacing pulse, as an atrial ER starts approximately 10 ms after pulse delivery, and it has much smaller amplitude than a ventricular ER (which starts around 60 ms following a ventricular pacing pulse). Because the atrial ER signal is small and arises quickly after the atrial pulse is delivered, it can be obscured by polarization artifacts from the atrial pacing pulse: the injection of the atrial pacing pulse leaves residual charge in the interface between the pacing electrode and the myocardial tissue.
Several techniques have been proposed for distinguishing ER signals from polarization artifacts. One approach is to use low-polarization coatings on the pacemaker electrodes, such as fractal iridium (Ir), that provide high Helmholtz double-layer capacities which assist in reducing polarization (and thus artifacts). However, the use of these coatings alone may not be sufficient to reduce artifacts to such an extent that the ER is easily discernable.
Another approach is to simply use different electrodes for pacing and for ER sensing, thereby isolating the polarization at the pacing electrode from the ER sensing electrode. This approach can be difficult to practically implement in a leadless pacemaker, where size/space is critical, thereby making it highly desirable to use the same electrodes for pacing and sensing. Thus, “passive” and “active” charge balancing methods have been developed to discharge the residual charge at the electrode/tissue interface, and better allow use of the same electrode for both pacing and sensing. In contrast to active charge balancing systems, wherein the residual charge is monitored and then battery current is used to negate it, passive charge balancing systems seek discharge without the need for battery current. U.S. Pat. No. 6,044,296 shows passive charge balancing method wherein an extra capacitor is switched in series with the output direct current (DC) blocking capacitor to more quickly discharge the polarization charge after a stimulus pulse. U.S. Pat. No. 8,224,446 uses a passive charge balancing method wherein after potentials resulting from the pacing pulse are attenuated by reducing coupling capacitance. U.S. Pat. No. 8,948,866 teaches a similar technique of minimizing the post-pacing artifact by using a smaller coupling capacitor borrowed from a backup pacing capacitor, or another coupling capacitor from a different pacing channel. This approach also has drawbacks, as the capacitors needed to discharge the residual charge occupy additional space, posing challenges for minimizing the size of the pacemaker.
Yet another approach uses alternative pacing waveforms which reduce polarization effects. U.S. Pat. No. 8,340,762 discusses use of a tri-phasic pulse generation technique to reduce the polarization effects, an approach originally proposed by U.S. Pat. Nos. 4,343,312 4,543,956 teaches utilizing a biphasic current pulse with automatic and dynamic compensation utilizing integrators. A disadvantage of these approaches is that they use current-based methods for stimulation pulses and for subsequent balancing (i.e., negation of polarization), and for better power efficiency and battery life, leadless pacemakers are preferably voltage-based stimulation devices, like traditional pacemakers.
Several references also teach analog and digital filtering methods for isolating ER signals from artifacts, e.g., U.S. Pat. Nos. 7,474,922 and 7,089,049. Many others disclose different signal post-processing techniques to eliminate the artifact, or discuss use of the polarity of the post-pacing response signal to confirm whether capture occurred or not (e.g. U.S. Pat. No. 6,865,421). These approaches also have drawbacks, as second order band-pass filters in the input circuit of a pacemaker may distort a polarization artifact to such an extent that it is falsely interpreted as an evoked response (ER). Special digital filtering techniques are also not preferred for implementation in a leadless pacemaker given the limited computing resources typically available.
Other approaches detect capture via detection of (mechanical) heart motion, rather than via detection of (electrical) ER. U.S. Pat. No. 5,549,652 describes sensing capture by detecting motion of the cardiac wall using a sensor present in a lead. Published international patent application WO 2005/089866 A1 proposes detection of capture in a cardiac cavity by detecting contraction from a signal representing endocardial acceleration (EA) delivered by an accelerometer sensor situated in a lead. U.S. Pat. No. 8,214,036 presents improvements to the detection described in international patent application WO 2005/089866 A1, with focus on atrial capture. U.S. Pat. Nos. 8,489,188 and 8,862,231 teach similar approaches based on EA signals. U.S. Pat. No. 8,801,624 describes use of an implantable heart sound sensor (e.g. an accelerometer) configured to initiate a paced cardiac contraction. Similarly, U.S. Pat. No. 8,923,963 describes a leadless atrial pacemaker having a mechanical sensor that generates signals indicative of contraction of a ventricle.
While some of the obstacles to achieving effective capture verification while at the same time furthering pacemaker miniaturization will undoubtably decrease as further developments are made in electronics miniaturization and in battery technology, it would nonetheless be useful to have additional approaches which at least reduce some of the drawbacks noted above. In particular, it would be useful to have effective capture verification methods available for use in leadless pacemakers designed for implantation in the atrium, where the electrical signals indicating capture are much more challenging to measure than in the ventricle.